make seamless textures is not so hard

As chicken said, making seamless textures is pretty easy. Only takes a few minutes.

Can I also make seamless textures in Paint.NET? Because Photoshop costs a lot...

Go check if it has an Offset and Clone functions.

Yes it has an offset plugin.
These are the cliffs with the new textures:

You may want to sculpt the edges and lines a bit better?

Thanks, it looks much better, after sculpting the cliff:

Inturesting, you still have some rather odd shading.. What does blender have for assigning smoothing groups? because this seams to be a smoothing error.

Other than that its certainly improving mate

In Blender smooth shading can be assigned for each face/polygon, but i cannot say how it compares to Max's smoothing groups.

I've used Google Sketchup this time to sculp the edges, what do you think?

Hmm, still have some odd Shading triangles. :/

Shaping and sculpting rocks of great detail requires alot of work and dedication. I would recommend searching Google Images of cave rocks and similar. Get some references from mother nature.

yes indeed! that's a good thing to do, nothings better than sampling the real thing! and if that doesn't work.. at least try sketching some basic rock shapes that can help with realism.. Of course the tools you use are important too.. have you tried a Zbrush or mudbox trial?

The left-side cliff looks good, but the right-side one still has some dodgy shading (hard edges) on some of the polygons. The texture is looking a lot improved in the new versions.

I've practiced a lot with texturing and I have created the cliff again, from the beginning, this is the result, I think the textures are now much better:
The black faces are just shadows.